1. Field of the Invention
The present invention relates to a doped mercury arc lamp, and also concerns various instruments, such as a flow cytometry apparatus, which utilize such a mercury arc lamp for operation.
2. Description of the Prior Art
Mercury arc lamps are used for optical instrumentation, photochemistry, spectroscopy, and various applications requiring high intensity ultraviolet and other wavelength radiation. The point source configuration of the arc renders the mercury arc lamp most suitable for the aforementioned applications. One specific field in which mercury arc lamps are utilized is flow cytometry. In a typical flow cytometry apparatus, cells, particles or other biological elements are caused to flow, in a liquid stream, single file past a beam of light. This beam of light intersects the flowing liquid stream containing the cells at a right angle (nominal). Light scattered by the particles passing through the light beam may be detected at different angles with respect to the axis of the beam of light. Various characteristics of these cells may be determined in conjunction with the detection of scattered light. Further, the cells may be inherently fluorescent or may be labeled with a fluorescent marker. As these fluorescent cells pass through the light at a stimulating frequency, the fluorescent cells are energized and emit fluorescence which may be detected with respect to each cell so energized. A mercury arc lamp is a desirable light source for providing the beam of light for such flow cytometry apparatuses. Representative flow cytometry apparatuses are described in U.S. Pat. Nos. 3,826,364; 4,284,412; and 4,348,107.
One of the problems experienced with mercury arc lamps used in optical instrumentation is the short operative life span. Life span of mercury arc lamps, as well as many other light sources, may be defined as the time it takes for the power output to fall below a certain level. For example, a mercury arc lamp rated at 100 watts may be said to have outlived its usefulness if, after 100 hours, its output has fallen below a certain level. It is then said to have a life rating of 100 hours. In flow cytometers, this minimum level is closely related to the number of dye molecules contained on a cell which need to be detected.
Presently known and available mercury arc lamps, suitable for optical instrumentation uses, have a reliable life rating of about 100 hours. This is a typical life rating for a 100 watt mercury arc lamp which emits radiation between 225 and 650 nanometers (nm). Across this spectrum of frequencies, the mercury arc lamp has strong lines or emission properties at a number of different wavelengths. For example, there may be strong lines at approximately 365, 436 and 546 nanometers. Of course, these numbers are merely exemplary and are not intended to limit the lines of a mercury arc lamp in any way.
A known mercury arc lamp, available from Advanced Radiation Corporation, Santa Clara, Calif., has sought to improve the life rating of the lamp. This known arc lamp includes a metallic reflective coating, usually gold-based, around the center portion of the glass envelope. The coating is positioned so as to substantially cover the cathode. It is understood that the reflective coating causes an increase in the lamp temperature so that the mercury contained within should vaporize properly and both increase arc stability and extend lamp life. However, a life rating of 100 hours is still a typical value for the mercury arc lamp with the single reflective coating. Representative mercury arc lamps are described in U.S. Pat. Nos. 3,593,056; 4,156,826; and 4,295,074.
There is, therefore, still a need for mercury arc lamps, useful for optical instrumentation, which have improved life ratings without compromising the power output particularly at the wavelengths of interest.